Archie Dent is convinced that he and his friends Hachi and Fergus are the first three members of a new League of Seven: a group of heroes who come together to fight the Mangleborn whenever the monsters arise to destroy humanity.

But that belief is put to the test when the three are forced to undertake separate missions. Archie and his faithful Tik Tok servant, Mr. Rivets, ride a ten-story-tall steam man to the west coast, chasing a shape-shifting girl who has stolen the Dragon Lantern–an ancient artifact with mysterious powers. Meanwhile, Hachi and Fergus travel to New Orleans to find the mysterious Madame Blavatsky, who may hold clues to how and why Hachi’s father was murdered.

Along the way, the three heroes meet more young people with extraordinary powers who may join the League of Seven. But will the deep-rooted secrets they uncover destroy the new League before it even comes together?